Optical methods have been used to measure the thickness of transparent materials for decades. Specifically, if one directs a small beam of radiation at a uniform smooth transparent plate, then detects and measures the location of the reflections originating from the inner and outer surfaces, at an angle away from normal to the surface, the spacial separation of the two reflections is a function of the angle of incidence and reflection, the optical index of refraction of the plate, and thickness of the plate at the point where the reflections occur. A summary of this technique is taught in U.S. Pat. No. 4,902,902 by Tole. If the transparent plate is not uniform, such that the two surfaces are not parallel, then the spacial separation of the two reflections is also a function of the angle between the two surfaces at the point of reflection and the distance at which the measurement is made.
As used herein, transparent means clear, translucent or partially transmitting such that a discernible image of the second surface reflection can be formed and detected, at some wavelength of electromagnetic radiation.
For many transparent articles or objects such as float glass, windows or glass containers and glass tubing, the parallelism, uniformity, or concentricity of the two surfaces cannot be well controlled for example because of viscosity variations in the plastic forming state. The undulations in the inner or outer surface can cause prism effects, which produce very significant errors in the thickness measurement based on the spacial separation of the two reflections. In this case, if a small collimated light source such as a Laser beam is used for illumination of a flat non-uniform plate, the inside surface reflected beam will not exit the outer surface parallel to the reflection from the outer surface. The spacial separation of the two reflections will then also be a function of the distance from the object to the measuring system and the surface wedge angle in the plane of the two light beams.
Collimated light is a beam of light for which the exiting rays are essentially parallel and does not appreciably change its cross-section area with increasing distance from the source. Non collimated light may be converging or diverging, or a combination of both on different axis.
A problem with collimated or converging source illumination is that it requires exact object surface placement to keep both of the surface reflections within the field of view of the detector. A typical embodiment of the two surface reflection techniques is to set a collimated illumination source and the detector array very close to the article or object being measured. The close proximity of the detector to the article will reduce the offset of the two reflected beams at the detector caused by surface tilt or from internal surface undulations. However, using a short optical path length does not correct for the internal prism error. Also, a close spacing between the wall thickness measurement device and the article being inspected makes it very difficult to measure non-flat or noncircular articles or objects such as flask shaped containers.
Another problem with collimated point source illumination such as a Laser beam, is that each measurement samples only a very small area of the article. A typical situation is provided by rotating a cylindrical article in a captive fixture, as taught by Juvinall et. al. U.S. Pat. No. 5,291,271, allows multiple point measurements to be taken over the entire circumference at one elevation. Measurements at one elevation may not be representative of thickness in the area, and a major thin or thick spot nearby could be missed if it occurs above or below the circumferential scan. The use of multiple scanning heads can provide additional measurements at different elevations, but the vast majority of the article surface is not inspected.